As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which exhibit high energy density and voltage, long lifespan and low self-discharge rate are commercially available and widely used.
As positive electrode active materials for such lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition thereto, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and the like, and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
LiCoO2 is widely used due to excellent overall physical properties such as excellent cycle properties, and the like, but is low in safety. In addition, due to resource limitations of cobalt as a raw material, LiCoO2 is expensive and massive use thereof as power sources in fields such as electric vehicles and the like is limited. Due to characteristics of preparation methods of LiNiO2, it is difficult to massively produce LiNiO2 at reasonable expense.
On the other hand, lithium manganese oxides, such as LiMnO2, LiMn2O4, and the like, are advantageous in that they contain Mn, which is an abundant and environmentally friendly raw material, and thus are drawing much attention as a positive electrode active material that can replace LiCoO2. However, such lithium manganese oxides also have disadvantages such as poor cycle characteristics and the like.
First, LiMnO2 has disadvantages such as a low initial capacity and the like. In particular, LiMnO2 requires dozens of charge and discharge cycles until a constant capacity is reached. In addition, capacity reduction of LiMn2O4 becomes serious with increasing number of cycles, and, at particularly high temperature of 50° C. or more, cycle characteristics are rapidly deteriorated due to decomposition of an electrolyte solution, elution of manganese and the like.
Meanwhile, as lithium-containing manganese oxides, there is Li2MnO3 in addition to LiMnO2 and LiMn2O4. Since structural stability of Li2MnO3 is excellent but it is electrochemically inactive, Li2MnO3 itself cannot be used as a positive electrode active material of secondary batteries. Therefore, some prior technologies suggest a technology of using a solid solution of Li2MnO3 and LiMO2 (M=Co, Ni, Ni0.5Mn0.5, Mn) as a positive electrode active material. In such a positive electrode active material solid solution, Li and O are separated from a crystal structure at a high voltage of 4.5 V and, thus, electrochemical activity is exhibited. However, there are problems such as high possibility of electrolyte solution decomposition and gas generation at high voltage, and massive use of relatively expensive materials such as LiMO2 (M=Co, Ni, Ni0.5Mn0.5, Mn) and the like.
In addition, due to structural characteristics of lithium-containing manganese oxide crystals, it is difficult to guarantee desired stability and limited to anticipate improvement of energy density.
Therefore, there is an urgent need for technology to resolve such problems.